LMTD Heat Exchanger Calculator
Log mean temperature difference, the heat duty it gives, and the area a 100 kW duty would need.
Results
Log mean temperature difference
36.4096
K
Power
218.4574
kW
Temperature difference at one end
40.0000
K
Area
5.4931
m²
What this tool does
The two inputs are the temperature differences between the hot and cold streams at each end of the exchanger. The log mean is always smaller than the plain average — 36.4 against 40 in the default case — because the driving force is not constant along the exchanger, and using the average overestimates the duty. The correction factor is 1 for pure counterflow and less than 1 for shell-and-tube and crossflow arrangements; take it from the published charts for the configuration.
Formula
ΔT ml = (ΔT₁ − ΔT₂) ÷ ln(ΔT₁ ÷ ΔT₂) ; Q = U A ΔT ml
Variables
| Symbol | Meaning | Unit |
|---|---|---|
d1 | Temperature difference at one end | K |
d2 | Temperature difference at the other end | K |
uu | Overall heat transfer coefficient | W/(m²·K) |
aa | Area | m² |
ft | Configuration correction factor | — |
LM | Log mean temperature difference | K |
QQ | Power | kW |
AM | Temperature difference at one end | K |
AR | Area | m² |
Worked example
- Temperature difference at one end60 K
- Temperature difference at the other end20 K
- Overall heat transfer coefficient500 W/(m²·K)
- Area12 m²
- Configuration correction factor1
- Log mean temperature difference36.4096 K
- Power218.4574 kW
- Temperature difference at one end40.0000 K
- Area5.4931 m²
Limitations
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.